Wireless communication system

ABSTRACT

A data signal is transmitted in a wireless communication system from a primary station to a secondary station. The data signal comprising a medium access control header (MAC-hs header) and a protocol data unit associated with the medium access control header. The medium access control header comprising a set at least one field (e.g. TSN, SEG, LI, FMT, LCid), wherein a size allocated to at least one field of the set is determined based on a length of the protocol data unit.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/618,031, filed on Feb. 10, 2015, which is a continuation ofapplication Ser. No. 12/522,049, filed Jul. 2, 2009 (now U.S. Pat. No.8,953,519), which is the National Stage of International Application No.PCT/IB2008/050028, filed Jan. 7, 2008, which claims the priority offoreign application EPO EP07300711 filed Jan. 9, 2007, all of which areincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The invention relates generally to a method of transmitting a datasignal in a wireless communication system from a primary station to asecondary station.

The invention also relates to a primary station for transmitting a datasignal to a secondary station, the primary and secondary station beingpart of such a wireless communication system.

The invention finds application, for example, in the field of mobilecommunication and more especially Universal Mobile TelecommunicationsSystem (UMTS) mobile communication.

BACKGROUND OF THE INVENTION

FIG. 1 shows a diagram of the protocol stack for UMTS. The radiointerface is layered into three protocol layers:

-   -   the physical layer (L1);    -   the data link layer (L2);    -   the network layer (L3).

A more detailed description is outlined in the 3GPP specificationdocument TS 25.301.

Layer 2 is split into following sub-layers: Medium Access Control (MAC),Radio Link Control (RLC), Packet Data Convergence Protocol (PDCP) andBroadcast/Multicast Control (BMC). Layer 3 and RLC are divided intoControl (C-) and User (U-) planes. PDCP and BMC exist in the U-planeonly. In the C-plane, Layer 3 is partitioned into sub-layers where thelowest sub-layer, denoted as Radio Resource Control (RRC), interfaceswith layer 2 and terminates in the UTRAN. Service Access Points (SAP)for peer-to-peer communication are marked with circles at the interfacebetween sub-layers. The SAP between MAC and the physical layer providesthe transport channels. The SAPs between RLC and the MAC sub-layerprovide the logical channels.

Also shown are connections between RRC and MAC as well as RRC and L1providing local inter-layer control services. These interfaces allow theRRC to control the configuration of the lower layers. For this purpose,separate Control SAPs are defined between RRC and each lower layer(PDCP, RLC, MAC, and L1). The RLC sub-layer can provide retransmissionfunctionality as well as segmentation/reassembly functionality. The MACsub-layer provides mapping of the logical channel(s) onto theappropriate transport channel(s), the selection of appropriate TransportFormat Combinations and the multiplexing/demultiplexing of higher layerProtocol Data Units (PDU) into/from transport channel PDUs (calledTransport Block). That means a MAC PDU is defined as a RLC PDU plus aMAC header.

The RLC provides three main delivery services to higher layers:

-   -   TM (Transparent Mode): does not add header information to higher        layer PDUs; however, it may segment the information if required,        with the size of the segments being determined from the        transport formats.    -   UM (Unacknowledged Mode): as TM above, but also allows for        concatenation of higher-layer PDUs; thus a header is required.    -   AM (Acknowledged Mode): provides segmentation and reassembly,        concatenation, error correction, in-sequence delivery of        higher-layer PDUs, duplicate detection, flow control, and        ciphering.

FIG. 2 shows the MAC header and data payload as currently defined forrelease 6 of UMTS in the Downlink (for High Speed Downlink Packet AccessHSDPA). The MAC header comprises:

-   -   VF (Version Flag) for extension capabilities;    -   Queue ID for identification of the reordering queue on the UE        (User Equipment) side;    -   TSN (Transmission Sequence Number) used for reordering purposes        (i.e. the received MAC PDUs are reordered according to their        associated TSN);    -   SID_(k) (Size Index iDentifier), which indicates the size of a        set of consecutive PDUs of the same length;    -   N_(k), which represents the number of consecutive PDUs;    -   F_(k), which is a flag indicating if SID_(k) or a PDU is        following (0=SID follows, 1=PDU follows).

Proposals have been made to design the MAC and RLC in release 7 of UMTSas follows. The RLC-AM (Radio Link Control Acknowledged mode) will beenhanced to support flexible PDU sizes (a PDU comprises data and controlinformation which is passed between layers in a protocol stack). TheMAC-hs (i.e. the MAC entity in the base station controlling the HighSpeed Downlink Shared CHannel HS-DSCH) will be enhanced to support RLCPDU segmentation. The SID and N fields (SID identifies logical channeland the size of N consecutive RLC PDUs and N the number of consecutiveRLC PDUs) need to be updated in order to be able to indicate the size ofthe RLC PDU. It seems highly likely that backwards compatibility will bemaintained by using the VF flag in the MAC-hs header.

The design of the RLC should take into account the available memory in agiven implementation for the processing of layer 2 data, the maximum RLCwindow size (namely the currently acceptable range of sequence numbersto the receiver) supported and the RLC round-trip-time. If the number ofRLC PDUs is increased for a given amount of data then the RLCtransmission window will advance faster and the receiver generate moreACK/NACKs (ACKnowledgement/Negative ACKnowledgement) in the statusreports. There has been a drive to reduce L2 RLC RTT to avoid RLC windowstalling. RLC window stalling is when the RLC entity has to wait (i.e.window cannot advance) for a re-transmission and positive acknowledgmentof a PDU. The maximum sustainable rate can be estimated by:Rate=window size*PDU Size/RTT (assuming zero RLC-level retransmissions).

With typical assumptions of a 2047 RLC window size (4095 is themaximum), a 100 ms round trip delay between RLC and UE and a 320 bit RLCPDU size, an error less flow would sustain a maximum data rate of2047*320/0.1=6.55 Mbps (13.1 Mbps with 640 bit PDUs).

As seen above, the maximum sustainable data rate of RLC would not behigh enough to support the expected data rates (greater than 14 Mbps)when MIMO or higher-order modulation (64 QAM) is used in Release-7 UMTS.Therefore, some improvements to layer 2 have to be made.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved methodof transmitting a data signal in a wireless communication system from aprimary station to a secondary station.

In accordance with a first aspect of the invention, there is provided amethod of transmitting a data signal in a wireless communication systemfrom a primary station to a secondary station, the data signalcomprising a header and a protocol data unit associated with the header,the header comprising a set of fields, wherein the method comprisesdetermining a size allocated to at least one field of the set on thebasis of a length of the protocol data unit.

Such a method reduces the amount of overhead in the header by optimisingthe fields comprised in the header as a function of the length of theprotocol data unit.

More specifically, this method proposes a flexible MAC header format,which allows optimisation for a range of possible RLC PDU sizes,multiplexing and segmentation. The invention allows the MAC header sizeto be reduced, based on the recognition that not all combinations offield-contents that may be signalled in the MAC header are useful in allscenarios, and the MAC header size may therefore be reduced by reducingthe size of at least one of the fields. In some cases, this reductionarises from the recognition of dependencies between different fields ofthe header: for example, it may be identified that in certain scenarioswhich require a first field to be able to signal a large range of values(requiring a large field size), the range of values required in a secondfield may be reduced, and therefore the size of the second field may bereduced.

According to an embodiment of the invention, the size allocated to asegmentation indication field determined for a first length of theprotocol data unit is lower than the one determined for a second lengthof the protocol data unit, the second length being higher than the firstlength, the segmentation indication indicating in how many segments theprotocol data unit is subdivided.

According to another embodiment of the invention, the size allocated toa transmission sequence number field determined for a first length ofthe protocol data unit is higher than the one determined for a secondlength of the protocol data unit, the second length being higher thanthe first length, the transmission sequence number being used forreordering the protocol data unit.

The invention is based on the recognition that, when protocol data unitsare large then typically a smaller range of transmission sequencenumbers is required, with more segmentation, as opposed to smallprotocol data units with a larger range of transmission sequence numberand little or no segmentation.

According to another embodiment of the invention, a value of a formatfield is determined on the basis of the length of the protocol dataunit, the format value indicating the size allocated to the at least onefield of the set.

Beneficially, the wireless communication system is the Universal MobileTelecommunications System.

In accordance with a second aspect of the invention, there is provided aprimary station for transmitting a data signal to a secondary station,the primary and secondary station being part of a wireless communicationsystem, the data signal comprising a header and a protocol data unitassociated with the header, the header comprising a set of fields,wherein the primary station comprises means for determining a sizeallocated to at least one field of the set on the basis of a length ofthe protocol data unit.

The primary station may be a mobile phone or a base station. The meansfor determining are, for example, a controller or a processor.

In accordance with a third aspect of the invention, there is provided adata signal for use in a wireless communication system, the data signalcomprising a header and a protocol data unit associated with the header,wherein the header comprises:

-   -   a length indication that indicates the length of the protocol        data unit,    -   a transmission sequence number used for reordering the protocol        data unit and/or a segmentation indication that indicates into        how many segments the protocol data unit is subdivided,

wherein the size allocated to one of: the length indication,transmission sequence number, and segmentation indication; is determinedon the basis of the length indication.

The invention finally relates to a computer program product directlyloadable into an internal memory of a primary station, comprisingsoftware code portions for performing all the steps of the method whenthe product is run on the primary station.

These and other aspects of the present invention will be apparent from,and elucidated with reference to, the embodiment described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way ofexample only and with reference to the accompanying drawings, in which:

FIG. 1 is a block a diagram of the protocol stack for UMTS,

FIG. 2 shows the structure of a MAC header in accordance with release 6of UMTS, and

FIGS. 3 to 6 show the structure of a MAC header in accordance withdifferent embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In Release-7 UMTS, in order to support higher data rates, the RLC-AM isbeing redesigned to be able to use flexible RLC PDU sizes. This willrequire a new MAC header to support the transmission of the RLC PDUsover HARQ (Hybrid Automatic Repeat Request) processes.

This invention proposes a flexible MAC header format, which allowsoptimisation for a range of possible RLC PDU sizes dependent on theclass of data and/or the data packet size to be transported. Thisincludes for example the RLC PDU size, and/or the available transportblock sizes, and/or the logical channel to be transported.

The invention takes the following aspect into consideration. If therange of Transmission Sequence Numbers TSNs that can be signalled is toosmall (i.e. too short for a Sequence Number field in the MAC header)then it may be possible that when many RLC PDUs are transmitted the TSNwraps around and therefore the receiving MAC entity cannot reconstructthe correct order for the RLC PDUs. On the other hand, if the range ofTSNs that can be signalled is too large then more bits are being usedthan are required and this increases the amount of signalling overhead,and thus reduces the efficiency of the overall data transmission.Similarly, if the number of segmentation bits is too small, the numberof segments into which an RLC PDU may be divided may be too limited forlarge packets, while if the number of segmentation bits is large theoverhead of the MAC header may be unnecessarily increased for small RLCPDUs which are not segmented. Therefore, an object of the invention isto reduce the MAC header overhead while still providing flexiblesegmentation and sequence numbering.

The invention is based on the fact that, when RLC PDUs are large thentypically a smaller TSN range is required, with more segmentation,whereas small RLC PDUs typically require a larger TSN range and littleor no segmentation.

FIG. 3 shows the MAC header structure in accordance with the invention.

The Release-6 field N (representing the number of consecutive MAC-dPDUs) is replaced by a Length Indicator LI, which works for example onbyte aligned basis allowing byte accuracy in MAC-hs segmentation. Thelength indicator may indicate the length of the whole RLC PDU (ifsegmentation is predetermined to take place into equally-sized segments)or the length of the segment of the RLC PDU contained within the currentMAC PDU. Additionally, a segmentation field is shown, comprised of, forexample, 2 bits, which could indicate up to 4 segments—the “firstsegment”, “first middle segment”, “second middle segment” and “lastsegment” of the MAC-hs SDU in a similar manner as in RLC from Release-6.Alternatively, only three of the available four values can be used ifthe segmentation is predetermined to subdivide the RLC PDU into onlythree segments.

In the lower half of FIG. 3, is shown an additional 2-bit format (FMT)field in the MAC header. This format is used to indicate 4 possiblecombinations of TSN and segmentation indication. In the above examplethe following 4 cases are shown:

-   -   Format 0 (e.g. 00) corresponds to a 5-bits TSN for the case        where no segmentation is performed and therefore small RLC PDUs        are assumed, and the maximum number of bits is therefore        required to signal the TSN to avoid window wrap-around.    -   Format 1 (e.g. 01) is where 2 bits are allocated to signalling        the segmentation of the RLC PDUs. In this case a smaller amount        of signalling is required for the TSN, as the RLC PDUs are        assumed to be larger.    -   Format 2 (e.g. 10) shows more segmentation being signalled and        thus the range of RLC PDUS transmission sequence numbers will be        even smaller than in Format 1.    -   Format 3 (e.g. 11) shows the extreme case where no sequence        number is defined and all the remaining bits are used for        Segmentation indication. Typically though it could be more        efficient in this case to also shorten the MAC header to reduce        the overall signalling load. This format may be appropriate for        certain data flows that do not require re-ordering, for example        highly delay sensitive traffic.

Alternatively and advantageously, rather than using a FMT field, theformat of the MAC header can be configured by RRC signalling. This wouldavoid increasing the MAC header overhead by adding the FMT field. Morespecifically, the RRC signalling alternative consists in pre-definingthe format in a semi-static way. For example when an initial connectionbetween a UE and a network is established, a procedure entails thesetting up of a radio bearer for the transmission of data in the user orcontrol planes. When this radio bearer is set up then the format of theMAC header is also set up at the same time for the data carried in thatradio bearer. This header format is then used for the duration of thelife of that radio bearer, typically as long as the radio bearer is inuse.

It may be also possible to have some combination of the use of the FMTfield and RRC signalling where RRC signalling indicates a range offormats and then a format indicator indicates a sub range of theidentified range.

According to an embodiment of the invention, a dependency is exploitedbetween the size of the RLC PDUs and the amount of segmentation SEGrequired. When the RLC PDU is segmented into many portions, thentypically larger RLC PDUs are being used, which require more bits tosignal the range of possible lengths, and when less segmentation isperformed, then the RLC PDUs will typically be smaller requiring asmaller range of lengths to be signalled. In summary:

-   -   Large RLC PDUs→large LI, large SEG,    -   Small RLC PDUs→small LI, small SEG.

Thus it may not be necessary to configure the length of the SEG fieldindependently from the length of the LI field, but the length of one maybe inferred from the length of the other.

In an extension of this embodiment, the Logical Channel ID (LCid) isused to indicate the size of the LI, as shown by the example in FIG. 4.For example, a range of LCids is defined where part of that range canindicate that a smaller size of LI is used, a smaller size of LI beingused typically when smaller PDUs are transmitted. In summary:

-   -   Large RLC PDUs→use first set of LCids→large LI, large SEG,    -   Small RLC PDUs→use second set of LCids→small LI, small SEG.

In the example shown in FIG. 4, five bits are used for the logicalchannel ID such that a range of 32 logical channels can be signalled. Iffor example, some of those 32 logical channels are typically onlycarrying smaller PDUs, then the amount of signalling required for thelength of that PDU is typically much smaller, thus the size of the fieldused to indicate length (the LI field) can be equally much smaller. Inthe example shown, when another range 3 of LCids are used then thelength indicator can be sent alone without a segmentation indication.

In a further embodiment, the length of the LI field can determine theinterpretation of the SEG field. For example, for large PDUs that maynot fit into a MAC-hs PDU, then segmentation is more likely to berequired. For smaller RLC PDUs, segmentation is generally less likely tobe required. However, when small RLCs are being assembled into a MAC-hsPDU, there may be space available for more than one of these RLC PDUs tobe transmitted in the same MAC-hs PDU. Then, the SEG field could bere-used as an indication of the number of consecutive RLC PDUs of thesame LCid that are concatenated into the MAC-hs PDU. As an example ofthis case, FIG. 5 indicates the two possible cases for the use of thesegmentation field. In the first example, the RLC PDU is segmented intoMAC-hs PDU 1 and MAC-hs PDU2 and the SEG field acts as a segmentidentifier, while in the second example, three smaller RLC PDUs(numbered 1, 2, 3) are concatenated into a MAC-hs PDU. In the secondcase, the SEG field is no longer an indication of segmentation, butbecomes used to indicate the exact number of RLC PDUs contained in theMAC-hs PDU (more like a multiplexing identifier). This can be summarisedas:

-   -   If LI length (or a LCid indicating a given LI length)>threshold        then SEG=indication of segmentation, and    -   If LI length (or a LCid indicating a given LI length)<threshold        then SEG=indication of number of consecutively concatenated RLC        PDUs, where threshold is a pre-defined measure of LI, where        above it, the size of RLC PDUs is generally large enough so that        segmentation rather than concatenation is required.

FIG. 6 shows another aspect of the embodiments of the invention, wherebyan additional 2 bit format (FMT) field is introduced into the MAC headerto indicate 4 possible combinations of length and segmentationindication. In the above example the following 4 cases are defined:

Format 0: 10 bits LI for the case where up to 3 bits of segmentation isperformed and therefore there is a large range of possible RLC PDU sizespossible, and the maximum number of bits is required to signal theLength indication and possible MAC segmentation.

Format 1 is where 2 bits are allocated to signalling the MAC levelsegmentation. In this case a smaller amount of signalling is requiredfor the Length Indication (9 bits).

Format 2 shows no segmentation being signalled and thus the range oflengths of RLC PDU sizes that are required to be signalled will be evensmaller (5 bits).

Format 3 shows the extreme case where only one length of RLC PDU isdefined all the no bits are used for length or segmentation indication.This reduces the size of the MAC header and thus the overall signallingload. This format may be appropriate for certain data flows that alwayshave a fixed length transmission packet size, where segmentation is notrequired.

Alternatively and as described hereinbefore, rather than using a FMTfield, the format of the MAC header could be configured by RRCsignalling.

It should be noted that an alternative implementation to the embodimentsalready described is the case where no segmentation field is defined inthe MAC header and only the size of the LI field varies.

One or more of the following factors may determine the choice of formatfor the new optimised MAC header:

-   -   the size of the maximum RLC PDU to be transmitted;    -   the logical channel to be transmitted, which could be configured        by RRC at when the logical channel is defined at radio bearer        setup; and    -   the available transport block size, which, if small may require        that more segmentation takes place to allow RLC PDUs to be        efficiently transmitted.

Note that while the above description is in terms of “downlink” (i.e. atransmission from a base station to a mobile station), the invention isequally applicable to the uplink (i.e. a transmission from a mobilestation to a base station).

It should be noted that, the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe capable of designing many alternative embodiments without departingfrom the scope of the invention as defined by the appended claims. Forexample, an embodiment of the invention relates to a method oftransmitting a data signal in a wireless communication system from aprimary station to a secondary station, the data signal comprising adata packet comprising a header and at least one protocol data unitand/or at least one part of a protocol data unit, the header comprisinga set of parameters wherein the method comprises determining a sizeallocated to a field for transmitting at least one parameter of the seton the basis of the size of at least one other field and/or the value ofat least one of the parameters. The set of parameters comprise one ormore of a transmission sequence number field, a segmentation indicationfield, a length indication field and a Logical Channel ID field.

In the claims, any reference signs placed in parentheses shall not beconstrued as limiting the claims. The word “comprising” and “comprises”,and the like, does not exclude the presence of elements or steps otherthan those listed in any claim or the specification as a whole. Thesingular reference of an element does not exclude the plural referenceof such elements and vice-versa.

The invention may be implemented by means of hardware comprising severaldistinct elements, and by means of a suitably programmed computer. In adevice claim enumerating several means, several of these means may beembodied by one and the same item of hardware. The mere fact thatcertain measures are recited in mutually different dependent claims doesnot indicate that a combination of these measures cannot be used toadvantage.

The invention claimed is:
 1. A wireless telecommunication devicecomprising: a processing circuit configured to encapsulate a radio linkcontrol (RLC) protocol data unit (PDU) with a media access control (MAC)header to generate a media access control (MAC) protocol data unit(PDU), the MAC header comprising a plurality of fields including aformat field which indicates a size of one or more of the plurality offields, the processing circuit being configured to determine a size ofthe radio link control (RLC) protocol data unit (PDU) and configure asize of a length indicator field of the header based on the determinedsize, wherein the size of the length indicator field is variable basedon the protocol data unit; and a transmitter configured to transmit themedia access control (MAC) protocol data unit (PDU) in a data signal. 2.The wireless telecommunication device of claim 1, wherein the wirelesstelecommunication device is one of a base station or a user equipment.3. The wireless telecommunication device of claim 1, wherein theplurality of fields further includes a logical channel identifier (LCID)field.
 4. The wireless telecommunication device of claim 1, wherein thewireless telecommunication device is configured to operate in aUniversal Mobile Telecommunications System (UMTS).
 5. The wirelesstelecommunication device of claim 1, wherein the format field indicatesthe size of the length indicator field.
 6. A wireless telecommunicationdevice, comprising: a receiver configured to receive a data signalincluding a media access control (MAC) protocol data unit (PDU), themedia access control (MAC) protocol data unit (PDU) comprising a header,the header comprising a plurality of fields including a format fieldwhich indicates a size of at least one of the plurality of fields, andwherein the plurality of fields further includes a length indicatorindicating a size of a length of the protocol data unit (PDU), whereinthe size of the length indicator field is variable based on the protocoldata unit; and a processing circuit configured to de-encapsulate themedia access protocol (MAC) protocol data unit (PDUC) from the datasignal based on the media access control (MAC) header to derive a radiolink control (RLC) protocol data unit (PDU).
 7. The wirelesstelecommunication device of claim 6, wherein the wirelesstelecommunication device is one of a base station or a user equipment.8. The wireless telecommunication device of claim 6, wherein theplurality of fields further includes a logical channel identifier (LCID)field.
 9. The wireless telecommunication device of claim 6, wherein thewireless communication device is configured to operate in a UniversalMobile Telecommunications System (UMTS).
 10. The wirelesstelecommunication device of claim 6, wherein the plurality of fields ofthe header further includes a transmission sequence number (TSN) fieldand a segmentation indication having a value indicating into how manysegments the protocol data unit is subdivided and wherein the formatvalue of the format field also indicates the size of the transmissionsequence number and of the value of the segmentation indication.